The present invention relates to an optical amplifier and in particular an optical amplifier suitable for use in a Wavelength Division Multiplexed (WDM) communications network.
In optically amplified WDM communications systems, to achieve acceptable signal-to-noise ratios (SNR) for all WDM channels it is necessary to have a constant value of gain for an to optical amplifier irrespective of the wavelength of any particular input channel. This is known as gain flatness and is defined as a low or zero value of the rate of change of gain with respect to wavelength at a fixed input level. Conventional amplifiers achieve gain flatness by careful design of Erbium Doped Fibre Amplifiers (EDFAs) and with the use of gain flattening filters. In addition, to ensure a constant output level from an optical amplifier when the input level changes most optical amplifiers also include an Automatic Level Control (ALC) circuit to adjust the gain by controlling the pump level. However. when the pump level is adjusted, the gain flatness of the amplifier is adversely affected exacerbating or introducing a gain tilt to the EDFA.
Fixed gain flattening filters may be used to address this problem. However, input level changes can be caused by cable ageing, cable repair or a fault in another EDFA in the amplifier. all of which may vary with time. Fixed gain flattening filters can not compensate the gain tilt changes that are caused by these variations. Alternative methods have been proposed to provide dynamic gain tilt adjustment, for example, making use of a variable gain equaliser or using an external attenuator coupled to the input of the EDFA. However, these devices are expensive and lead to an inevitable loss of gain of the amplifier over time.
According to a first aspect of the present invention, an optical amplifier for a wavelength division multiplexed communications system, comprises:
a first amplifier stage responsive to changes in gain to cause a wavelength dependent gain tilt in one direction:
a second amplifier stage responsive to corresponding changes in gain to cause a wavelength dependent gain tilt in an opposite direction; and,
gain control means for controlling the gain of the first and second amplifier stages to maintain a substantially flat response for the amplifier over time in which the first amplifier stage is a length of optical transmission fibre providing a Raman amplifier.
The present invention provides an optical amplifier which uses dynamic gain tilt adjustment whilst maintaining a constant output level from the amplifier. As such, the amplifier is particularly suitable for use in an optical repeater in a submarine communications network in which changes in the input level can occur as a result of many unpredictable factors as discussed above.
Preferably, the gain control means comprises a signal processing circuit coupled to an output of the optical amplifier arranged to detect changes in output level of the amplifier and adjust the gains of the first amplifier stage and the second amplifier stage accordingly and the signal processing circuit comprises a pump control circuit arranged in a negative feedback control loop around at least one amplifier stage. such that when a decrease in output level of the amplifier is detected the gains of the first amplifier stage and the second amplifier stage are adjusted accordingly to maintain the output of the amplifier at a substantially constant level.
As well as maintaining the output level of the amplifier at a constant level, the gain tilt of the amplifier with respect to input signal wavelength is maintained as substantially flat.
Preferably, the gain control means comprises at least one pump source arranged to provide pump light to the first amplifier stage and to the second amplifier stage, the power of the pump light being controlled in dependence on the output level of the amplifier.
More preferably, the amplifier further comprises an optical splitter arranged to couple a predetermined proportion of the pump light to the first amplifier stage thereby causing a wavelength dependent gain tilt in a first direction and a predetermined proportion of the pump light to the second amplifier stage thereby causing a wavelength dependent gain tilt in an opposite direction.
The overall increase in gain of the amplifier which is required can be provided by an increment in gain of the first amplifier stage and an increment in gain of the second amplifier stage. Therefore. the output level of the amplifier will be maintained and no undesired gain tilt will be introduced into the amplifier response.
According to a second aspect of the present invention, an optical repeater for a submarine communications system comprises an optical amplifier according to the first aspect of the present Invention.
According to a third aspect of the present invention, a submarine communications system comprises at least one optical repeater according to the second aspect of the present invention.